Articles | Volume 17, issue 17
Atmos. Chem. Phys., 17, 10767–10794, 2017

Special issue: The Saharan Aerosol Long-range Transport and Aerosol-Cloud-interaction...

Atmos. Chem. Phys., 17, 10767–10794, 2017

Research article 13 Sep 2017

Research article | 13 Sep 2017

Triple-wavelength depolarization-ratio profiling of Saharan dust over Barbados during SALTRACE in 2013 and 2014

Moritz Haarig1, Albert Ansmann1, Dietrich Althausen1, André Klepel1,2, Silke Groß3, Volker Freudenthaler4, Carlos Toledano5, Rodanthi-Elisavet Mamouri6, David A. Farrell7, Damien A. Prescod7, Eleni Marinou8, Sharon P. Burton9, Josef Gasteiger10, Ronny Engelmann1, and Holger Baars1 Moritz Haarig et al.
  • 1Leibniz Institute for Tropospheric Research, Leipzig, Germany
  • 2Technology Innovation Center, Goldschmidt Thermit GmbH, Leipzig, Germany
  • 3German Aerospace Center, Institute of Atmospheric Physics, Oberpfaffenhofen, Germany
  • 4Meteorological Institute, Ludwig Maximilians University, Munich, Germany
  • 5Group of Atmospheric Optics, University of Valladolid, Valladolid, Spain
  • 6Department of Civil Engineering and Geomatics, Cyprus University of Technology, Limassol, Cyprus
  • 7Caribbean Institute for Meteorology and Hydrology, Bridgetown, Barbados
  • 8Inst. for Astronomy, Astrophysics, Space Appl. and Remote Sensing, National Observatory Athens, Athens, Greece
  • 9NASA Langley Research Center, MS 475, Hampton, VA 23681, USA
  • 10Aerosol Physics and Environmental Physics, University of Vienna, Vienna, Austria

Abstract. Triple-wavelength polarization lidar measurements in Saharan dust layers were performed at Barbados (13.1° N, 59.6° W), 5000–8000 km west of the Saharan dust sources, in the framework of the Saharan Aerosol Long-range Transport and Aerosol-Cloud-Interaction Experiment (SALTRACE-1, June–July 2013, SALTRACE-3, June–July 2014). Three case studies are discussed. High quality was achieved by comparing the dust linear depolarization ratio profiles measured at 355, 532, and 1064 nm with respective dual-wavelength (355, 532 nm) depolarization ratio profiles measured with a reference lidar. A unique case of long-range transported dust over more than 12 000 km is presented. Saharan dust plumes crossing Barbados were measured with an airborne triple-wavelength polarization lidar over Missouri in the midwestern United States 7 days later. Similar dust optical properties and depolarization features were observed over both sites indicating almost unchanged dust properties within this 1 week of travel from the Caribbean to the United States. The main results of the triple-wavelength polarization lidar observations in the Caribbean in the summer seasons of 2013 and 2014 are summarized. On average, the particle linear depolarization ratios for aged Saharan dust were found to be 0.252 ± 0.030 at 355 nm, 0.280 ± 0.020 at 532 nm, and 0.225 ± 0.022 at 1064 nm after approximately 1 week of transport over the tropical Atlantic. Based on published simulation studies we present an attempt to explain the spectral features of the depolarization ratio of irregularly shaped mineral dust particles, and conclude that most of the irregularly shaped coarse-mode dust particles (particles with diameters > 1 µm) have sizes around 1.5–2 µm. The SALTRACE results are also set into the context of the SAMUM-1 (Morocco, 2006) and SAMUM-2 (Cabo Verde, 2008) depolarization ratio studies. Again, only minor changes in the dust depolarization characteristics were observed on the way from the Saharan dust sources towards the Caribbean.

Short summary
Our measurements performed with a lidar on Barbados give a vertical profile of Saharan dust, which was transported over 5000 km across the Atlantic. The new triple-wavelength depolarization technique reveals more information about the shape and size of dust, which will improve our understanding of the aging process of dust in the atmosphere and its representation in dust models. Changing properties of dust particles influence the solar radiation and the cloud properties and thus our climate.
Final-revised paper